Glass substrate for information recording medium and method for manufacturing the same

ABSTRACT

The present invention relates to a method for manufacturing a glass substrate for an information recording medium having a high level of cleanness and superior smoothness. The manufacturing method includes a step for washing a disk-shaped glass plate with an acid washing liquid, a step for removing at least part of a surface layer, which is formed on the surface of the glass plate, by performing grinding with diamond abrasion grains, and a step for washing the surface with a neutral or alkaline washing liquid.

This is a continuation application of U.S. patent application Ser. No.10/532,564 filed Aug. 17, 2005 which was a national stage entry of PCTApplication No. PCT/JP2003/013460 filed Oct. 22, 2003, which claimedpriority to Japanese Patent Application No. 2002-308811, filed Oct. 23,2002. The contents of each of these application is expresslyincorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a glass substrate for an informationrecording medium, such as a magnetic disk, a magneto-optical disk, or anoptical disk, and a method for manufacturing the same. Moreparticularly, the present invention relates to a glass substrate havinga surface in which a texture extending in the circumferential directionis formed and to a method for manufacturing the same.

BACKGROUND ART

A magnetic disk for a hard disk drive is known as one type ofinformation recording medium. The magnetic disk is a disk having acenter hole and is fabricated by superimposing magnetic films on thesurface of a glass substrate. The magnetic disk is rotated by a spindlereceived in the center hole. Information recorded on the magnetic diskis read by a magnetic head, which moves along the surface of themagnetic disk in a state levitated from the surface by a certaindistance.

It is desirable for the magnetic disk to have a high recording densityto increase the recording capacity of the magnetic disk. JapaneseLaid-Open Patent Publication No. 2002-150547 describes a method forsmoothing the surface of a glass substrate to decrease the distancebetween the magnetic disk surface and the head and increase therecording density of the magnetic disk. More specifically, an abrasivethat chemically affects the glass substrate, such as cerium oxide, isused to polish and smooth the surface of the glass substrate. An acidsolution is used to remove foreign articles such as iron particles andabrasive particles adhered to the smooth surface (acid washing). Then,the surface of the glass substrate is etched with an alkaline solution(alkaline washing). The etching removes about 10 nm of the glasssubstrate.

Due to the demand for a magnetic disk having a higher recording density,there is a tendency for further decreasing the distance between thesurface of the magnetic disk and the magnetic head. However, with themagnetic disk manufactured through the conventional method, the magnetichead cannot move further closer to the magnetic disk. More specifically,when alkaline washing removes about 10 nm of the glass substrate, themanufactured glass substrate may have deficiencies such as the etchingbeing uneven, protuberances of abnormal heights being produced on theglass substrate, and the surface state differing locally (differences inthe shapes of valleys and peaks). A magnetic disk manufactured from adeficient glass substrate has a tendency of causing a deficiency (glideerror) such as the moving head crashing against or being caught by anabnormal protuberance.

To solve this problem, a weak acid solution or weak alkaline solutionmay be used when performing the washing. However, many iron particlesand abrasive particles are chemically and firmly adhered to or caught inthe surface of the glass substrate. Thus, all of the iron particles andabrasive particles cannot be washed off when using a weak acid solutionor weak alkaline solution. In some cases, this would lower the cleannessof the glass substrate.

As another way to remove the iron particles and abrasive particles, thesurface of the glass substrate may be polished with an abrasive afterthe acid washing. However, this method is not preferable in that theabrasion grains contained in the abrasive may adhere to the glasssubstrate and in that the surface of the glass substrate may be deformedby performing washing after the polishing.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a glass substratehaving a smooth surface and a high cleanness.

One aspect of the present invention provides a glass substrate for aninformation recording medium formed from a disk-shaped glass plate madeof a multi-component glass material containing at least silicon oxide.The glass plate includes a surface layer with an ingredient ratio ofsilicon oxide that is higher than an inner portion of the glass plate.

Another aspect of the present invention is a method for manufacturing aglass substrate for an information recording medium. The manufacturingmethod includes a first washing step for washing a surface of adisk-shaped glass plate with an acid washing liquid. The first washingstep forms a surface layer on the surface of the glass substrate. Themethod further includes a step for grinding at least part of the surfacelayer with diamond abrasion grains to remove at least part of thesurface layer, and a second washing step for washing the surface with aneutral or alkaline washing liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a glass substrate according to anembodiment of the present invention.

FIG. 2A is a partial cross-sectional view showing the surface of a glassplate subsequent to texture formation.

FIG. 2B is a partial cross-sectional view showing the surface of theglass substrate of the embodiment.

FIG. 3 is a flowchart showing a process for manufacturing a glasssubstrate according to an embodiment of the present invention.

FIG. 4 is a perspective view showing the glass plate undergoing aprocess for forming a texture.

FIG. 5A is a cross-sectional view showing the glass plate in which thesurface layer is formed.

FIG. 5B is a graph showing the relationship between the depth andcomposition of the glass substrate.

BEST MODE FOR CARRYING OUT THE INVENTION

A glass substrate and a method for manufacturing the same according toan embodiment of the present invention will now be described.

As shown in FIG. 1, an information recording medium glass substrate 21is a disk having a center hole 21 b. The glass substrate 21 is made of amulti-component glass material that contains silicon oxide and at leastone of aluminum oxide and alkaline earth metal oxide.

Examples of multi-component glass materials include soda lime glass,aluminosilicate glass, borosilicate glass and crystallization glass thatare fabricated through a float process, a down draw process, a redrawprocess, or a pressing process. The main components of soda lime glassinclude silicon dioxide (SiO₂), sodium oxide (Na₂O), and calcium oxide(CaO). The main components of aluminosilicate glass include SiO₂,aluminum oxide (Al₂O₃), and R₂O (R is potassium (K), and sodium (Na) orlithium (Li). Examples of a crystallization glass include lithium oxide(Li₂O)—SiO₂ glass, Li₂O—Al₂O₃—SiO₂ glass, and RO—Al₂O₃—SiO₂ glass. ROrepresents alkaline earth metal oxide, and R represents magnesium (Mg),calcium (Ca), strontium (Sr), or barium (Ba).

A chemical strengthened glass fabricated by adding zirconium oxide(ZrO₂) or titanium oxide (TiO₂) to soda lime glass, aluminosilicateglass, borosilicate glass, or crystallization glass may be used as themulti-component glass material.

A plurality of films including a protective film and a magnetic film,which is made of metals or alloys of, for example, cobalt (Co), chromium(Cr), and iron (Fe), are formed on the surface 15 of the glass substrate11 to manufacture the information recording medium. The informationrecording medium is rotatably supported in an information recorder suchas a hard disk drive. The information recorder includes a head thatrecords information on the information recording medium and readsinformation from the information recording medium.

The head moves along the surface of the rotating information recordingmedium to a position where the desired recording information is recorded(seek operation). To prevent noise from being produced and deficiencies,such as scratching of the information recording medium, from occurring,the ideal seek operation is performed in a state in which the head islevitated from the surface of the information recording medium.Accordingly, the glass substrate 21 must have high smoothness. Due tothe increasing recording density over these recent years, it is requiredthat the levitation height of the head from the surface of theinformation recording medium (touch down height, TDH) be less than 5 nm.The head momentarily contacts the surface of the information recordingmedium during the seek operation.

The surface roughness of the glass substrate 21, more specifically, therange of the arithmetic mean roughness Ra regulated under JIS B 0601 ispreferably 0.1 to 1.5 nm, more preferably 0.1 to 1.0 nm, and mostpreferably 0.1 to 0.6 nm. The surface roughness Ra is measured using anatomic force microscope (AFM: manufactured by Digital Instruments,Inc.). If the surface roughness is greater than the above range, thesurface 22 of the glass substrate 21 becomes rough, the smoothnessbecomes low, and deficiencies (glide errors) such as the head crashingagainst or being caught by protuberances formed on the surface 22 duringthe seek operation tend to occur. If the surface roughness Ra is lessthan 0.1 nm, the polishing time for manufacturing the glass substrate 21is lengthened thereby decreasing yield and increasing the manufacturingcost of the glass substrate 21.

The maximum peak height Rp, which is regulated under JIS B 0601 andmeasured by an AFM, of the glass substrate 21 is preferably 10 nm orless. When the maximum peak height Rp exceeds 10 nm, abnormally tallprotuberances (asperities) of the surface 22 of the glass substrate 21tend to cause glide errors. Thus, the TDH cannot be decreased.

The ratio of Ra and Rp (Rp/Ra ratio) is preferably 10 or less. If theRp/Ra ratio exceeds 10, the surface roughness becomes uneven and itbecomes difficult for the head to pass over protuberances and asperitiesthereby resulting in glide errors apt to occurring.

The surface 22 of the glass substrate 21 has a texture 23 including aplurality of projections 24. The projections 24 concentrically extend inthe circumferential direction of the glass substrate 21. As shown inFIG. 2B, the peaks of the projections 24 are formed so that they do notexceed a reference line 25. Thus, the glass substrate 21 has superiorsmoothness, the head does not crash laterally against or become caughtby the projections 24, and the occurrence of glide errors is suppressed.

When used as an information recording medium, the glass substrate(textured glass substrate) 21, of which surface 22 includes the texture23, has a head contact area that is smaller than that of a glasssubstrate having a smooth or extremely smooth surface. Thus, thetextured glass substrate 21 suppresses adhesion between the surface ofthe information recording medium and the head that would be caused by aviscous material, such as lubrication oil, applied to the surface of theinformation recording medium.

In this manner, sticking and glide errors may be suppressed. Thisenables the TDH to be further decreased. In other words, the glasssubstrate 21 is optimal for increasing the recording density of theinformation recording medium.

A method for manufacturing the glass substrate 21 will now be described.

FIG. 3 is a flowchart showing the method for manufacturing the glasssubstrate 21. The manufacturing method includes a disk machining processS11, an edge chamfering process S12, a polishing process S13, apre-grinding washing process (first washing) S14, a grinding processS15, and a post-grinding washing process (second washing) S16.

In the disk machining process S11, a cutter made of cemented carbidealloy or diamond is used to cut a sheet of a multi-component glassmaterial and obtain a glass plate 21 a having a center hole 21 b.

In the edge chamfering process S12, the glass plate 21 a is ground toobtain the predetermined outer diameter and inner diameter. Further, thecorners at the inner and outer circumferences are chamfered byperforming polishing.

In the polishing process S13, the surface of the glass plate 21 a ispolished and smoothed. It is preferred that the polishing process S13 beperformed in two stages, a former stage polishing process and a latterstage polishing process. The former stage polishing process removeswarps, undulations, and deficiencies such as irregularities and cracks,so that the surface of the glass plate 21 a is flat and has an eventhickness. In the former stage polishing process, an abrasive of whichgrain diameter is relatively coarse is used. Further, either a polishingpad is not used or one that is hard and coarse is used.

In the latter stage polishing process, the glass substrate is polishedso as to satisfy the surface smoothness required for an informationrecording medium. The surface roughness of the glass plate 21 asubsequent to the latter stage polishing process is equivalent to thatof the glass substrate 21. That is, the glass plate 21 a is polisheduntil the arithmetic mean roughness Ra becomes 1.5 nm or less.

The abrasive used in the latter stage polishing process has a relativelyfine grain diameter and high affinity with respect to glass material andis, for example, a rare earth oxide, such as cerium oxide or lanthanumoxide, or colloidal silica. It is preferred that the polishing pad bemade of a soft and fine material such as, synthetic resin foam or suede.

The former stage polishing process and the latter stage polishingprocess may each be divided into further plural stages to improve thepolishing efficiency and surface smoothness of the glass plate 21 a.

In the pre-grinding washing process S14, the surface of the polishedglass plate 21 a is washed with a washing liquid. This removes adheredsubstances, such as the abrasive of cerium oxide or colloidal silicathat is chemically adhered to the surface in a firm manner and ironparticles caught in the surface. In the pre-grinding washing processS14, the glass plate 21 a is first immersed in a strong acid washingliquid (strong acid solution). This dissolves only the adheredsubstances, such as the abrasive or iron particles, or part of thesurface of the glass plate 21 a together with the adhered substances inthe strong acid solution and removes most of the adhered substances fromthe surface of the glass plate 21 a. Then, the glass plate 21 a isimmersed in a strong alkaline solution (strong alkaline washing liquid).This charges the glass plate 21 a and the adhered substances on theglass plate 21 a to the same polarity to cause electrostatic repulsionand removes the adhered substances from the glass plate 21 a.

As shown in FIG. 5A, immersion in the strong acid solution in thepre-grinding washing process S14 deforms the glass. The composition of alayer (surface layer) 27 near the surface of the deformed glass plate 21differs from the composition of an inner portion 26 of the glass plate21, or the portion 26 excluding the surface layer 27. The chemicalresistance of the surface layer 27 is lower than that of the innerportion 26.

The reason the surface layer 27 is formed will now be described.

Contact with the strong acid solution causes alkaline earth metal oxidesand aluminum oxides near the surface of the glass plate 21 a to dissolveinto the strong acid solution as alkaline earth metal ions and aluminumions. The ion radii of the alkaline earth metal ions and aluminum ionsare relatively large. Thus, large voids are formed in the molecularframe of the glass at the surface of the glass plate 21 a from which thealkaline earth metal ions and aluminum ions are removed. When suchsurface of the glass plate 21 a comes into contact with chemicals, suchas the acid solution or the alkaline solution, other ions derived fromthe chemicals enter the voids thereby affecting the Si—O bonding in theglass molecules near the surface. Accordingly, the chemical resistance,that is, acid resistance and alkaline resistance, of the surface layer27 is decreased.

After contact with the acid washing liquid, processing is performed withthe alkaline washing liquid to adjust the thickness and deformationlevel of the surface layer 27. Accordingly, the surface layer 27 is notformed with excessive thickness. More specifically, contact with thestrong alkaline solution uniformly etches the surface layer having lowchemical resistance to remove the excessively deformed portion of thesurface layer 27 and remove part of the surface layer 27 to obtain thedesired thickness.

Change in the immersion time of the surface layer 27 in the washingliquid adjusts the penetration level of the strong acid and strongalkaline solutions in the glass plate 21 a. This adjusts the thicknessand deformation level of the surface layer 27.

It is preferred that strong acid solution having a pH of 3.0 or less beused. When the pH exceeds 3.0, adhered substances cannot be sufficientlyremoved from the surface of the glass plate 21 a, and a glass plate 21 ahaving high cleanness cannot be obtained. As the strong acid liquid, atleast one selected from hydrofluoric acid, fluosilicic acid, sulfuricacid, hydrochloric acid, sulfamic acid, acetic acid, tartaric acid,citric acid, gluconic acid, malonic acid, and oxalic acid may be used.

It is preferred that a strong alkaline solution having a pH of 10.5 orgreater be used. If the pH is less than 10.5, the removal of the adheredsubstances from the surface of the glass plate 21 a becomesinsufficient. Further, uniform etching of the surface layer 27 becomesdifficult. As the strong alkaline solution, at least one selected from anon-organic alkaline solution, such as a potassium hydroxide solution, asodium hydroxide solution, or ammonia water, and an organic solution,such as tetraammonium hydride, may be used.

FIG. 5B is a graph showing depths from the surface of thealuminosilicate glass subsequent to the pre-grinding washing process andthe number of ions for each type of component measured by a secondaryion mass spectrometer (SIMS).

It is apparent from the measurement results that the number of calciumions (Ca²⁺) and magnesium ions (Mg²⁺), which are alkaline earth metalions, and aluminum ions (Al³⁺) decreases at positions deeper from thesurface of the glass plate 21 a. That is, the calcium ions, magnesiumions, and aluminum ions in the surface layer 27 are less than that inthe inner portion 26. As for silicon ions (Si⁴⁺) derived from siliconoxide, the number of ions is the same in the inner portion 26 and thesurface layer 27. Accordingly, the content of silicon oxide in thesurface layer 27 is relatively increased with respect to the innerportion 26 by the decrease of Ca²⁺, Mg²⁺, and Al³⁺.

More specifically, it is preferred that the ingredient ratio of siliconoxide in the glass composition of the surface layer 27 relative to theingredient ratio of silicon oxide in the glass composition of the innerportion 26 be greater by more than 1.0 times but less than or equal to1.2 times. If the ingredient ratio of silicon oxide in the surface layer27 becomes greater than the ingredient ratio of the inner portion 26 by1.2 times, the chemical resistance excessively decreases. Further, whenthe glass plate 21 a is immersed in the strong alkaline solution, thesurface of the glass plate 21 a is not uniformly etched and becomesrough. This may decrease smoothness.

In the grinding process S15, the surface of the glass plate 21 a isground to remove at least part of the surface layer subsequent to thewashing process S14. Further, the texture 23 is formed in the grindingprocess S15. In the grinding process S15, a texture machine, which isnormally used to perform texture processing on an aluminum substrate, isused.

The texture machine will now be described.

As shown in FIG. 4, a roller 31 is rotatably supported immediately abovethe glass plate 21 a. The roller 31 has a length that is substantiallyequal to the radius of the glass plate 21 a and extends in the radialdirection of the glass plate 21 a. A tape 32, which functions as a scrubmember, is arranged between the roller 31 and the glass plate 21 a topass from one side of the roller 16, into the space between the glassplate 21 a and the roller 31, and out of the other side of the roller16. The pressure of the roller 31 presses the tape 32 against thesurface 15 of the glass plate 21 a as the tape 32 passes through thespace between the glass plate 21 a and the roller 31. Further, anabrasive 33 is dropped on the surface of the glass plate 21 a. As theglass plate 21 a rotates in the direction of the arrow in FIG. 4, thetape 32 slides along the surface of the glass plate 21 a, and thesurface is ground while being controlled in a satisfactory manner toform the texture 23.

The material of the tape 32 is not particularly limited, and anymaterial, such as a tape-shaped cloth, non-woven cloth, or flockedarticle of polyethylene fibers or the like may be used as long as it canbe used to form such type of texture. The abrasive 33 is obtained bydispersing abrasion grains in a dispersion solvent such as water. Inaddition to rare earth oxides and colloidal silica, diamond abrasiongrains may be used as the abrasion particles. The preferred abrasiongrains are diamond abrasion grains that do not easily adhere to thesurface of the glass plate 21 a and do not chemically affect the glassplate 21 a. The grain diameter and shape of the diamond abrasion grainsis determined in accordance with the required density of the texture 23.The average grain diameter (D₅₀) of the diamond abrasion grains ispreferably 0.05 to 0.3 μm, and more preferably 0.08 to 0.25 μm. If D₅₀is less than 0.05 μm, the capability of polishing the glass plate 21 ais insufficient. This decreases the yield of the glass substrate 21 andincreases the processing cost. If D₅₀ exceeds 0.3 μm, projections 24having a large height difference are formed. This roughens the surfaceof the glass plate 21 a.

The texture machine rotates the glass plate 21 a in the direction of thearrow in FIG. 4 so that the tape 32 slides along and grinds the surfaceof the glass plate 21 a. As shown in FIG. 2A, most of the surface layer27 is removed from the glass plate 21 a after the grinding that formsthe texture 23, which includes the projections 24 on the surface. Thesurface layer 27 that was not removed remains on the upper portion ofsome of the projections 24.

This thickness of the surface layer 27 remaining on the surface of theglass plate 21 a after the grinding process S15 is preferably 3 nm orless. If the residual surface layer 27 is too thick, the surface layer27 will be unevenly etched in the post-grinding washing process S16 thusroughening the surface 22 of the glass substrate 21. The lower limit ofthe thickness of the residual surface layer 27 is 0 nm.

In the grinding process S15, the removal thickness (grinding amount) ispreferably 0.5 nm or greater. When the grinding amount is less than 0.5nm, the surface layer 27 remaining on the surface of the glass plate 21a after the grinding has a thickness of more than 3 nm. This roughensthe surface 22 of the glass substrate 21, which is obtained as describedabove. The grinding amount refers to the amount calculated bysubtracting the thickness of the glass plate 21 a subsequent to grindingfrom the thickness of the glass plate 21 a prior to grinding. Thus, thegrinding amount as used here does not refer to an amount representingthe average height of the projections 24 forming the texture 23. Theupper limit of the grinding amount is equal to the thickness of thesurface layer 27 immediately after the pre-grinding washing process S14.When the grinding amount exceeds the thickness of the surface layer 27,the grinding for removing the surface layer 27 may scratch the surfaceof the glass plate 21 a. This would lower the smoothness of the glasssubstrate 21.

In the post-grinding washing process S16, abrasion grains and dust isremoved from the surface of the glass plate 21 a to increase thecleanness of the glass substrate 21. For example, the glass plate 21 amay be immersed into a washing liquid so that abrasion grains and dustare washed off from the surface of the glass plate 21 a and dispersed inthe washing liquid.

In the post-grinding process S16, a neutral or alkaline washing liquidis used so that the glass plate is not chemically affected by theliquid. The alkaline solution used for process S14 may be used as thealkaline washing liquid. Examples of a neutral washing liquid are water;pure water; alcohol such as isopropyl alcohol; electrolytic waterobtained by performing electrolysis on a solution of a non-organic saltsuch as an alkaline metal salt like sodium chloride; or a neutralsolution such as functional water like gas dissolved water in which gasis dissolved. There are two types of electrolytic water, one obtained atthe anode side during electrolysis and the other obtained at the cathodeside. Any of these two types may be used as the washing liquid.

As described for the grinding process, to prevent the surface 22 of theglass substrate 21 from becoming rough, the surface layer 27 having athickness of 3 nm or less remains on the surface of the glass plate 21 athat has undergone grinding. To remove the surface layer 27 in asubstantially complete manner in the post-grinding washing process S16,it is preferred that an alkaline washing liquid be used to etch only thesurface layer 27 and prevent the inner portion 26 from being affected bythe liquid. The preferred washing liquid is an alkaline washing solutionhaving a pH of 11.0 to 13.0. If the pH is less than 11.0, the surfacelayer 27 may not be sufficiently removed. If the pH exceeds 13.0, theinner portion 26 may be etched in addition to the surface layer 27.Further, the surface layer 27 may not be evenly etched therebyroughening the surface 22.

To improve the washing effect in the post-grinding washing process S16,a builder, such as a surfactant, a chelating agent, and an organicsolvent, may be added to the washing liquid.

As described above, when using an alkaline washing liquid in thepost-grinding washing process, the surface layer 27 has a low chemicalresistance. Thus, the surface layer 27 is selectively dissolved andremoved by the washing liquid. As shown in FIGS. 2A and 2B, the surfacelayer 27 remains on the upper portion of the projections 24. Thisenables the height of each projection 24 to be the same as the referenceline 25, which is the boundary between the surface layer 27 and theinner portion 26. Further, the abrasive, iron particles, and abrasiongrains caught in or firmly adhered to surface of the surface layer 27are completely removed in the post-grinding washing process S16.Accordingly, the surface of the glass substrate 21 according to thepresent invention has both high cleanness and superior smoothness.

The embodiment has the advantages described below.

In the method for manufacturing the glass substrate 21, the diskmachining process S11, the edge chamfering process S12, the polishingprocess S13, the pre-grinding washing process S14, the grinding processS15, and the post-grinding washing process S16 are sequentiallyperformed. The strong acid solution used in the pre-grinding washingprocess S14 removes adhered substances from the surface of the glassplate 21 a and forms the surface layer 27 so that it has low chemicalresistance. During the grinding process, the surface of the glass plate21 a is ground so that the surface layer 27 is thinner than thepredetermined thickness. This prevents the washing liquid from unevenlyetching the surface layer 27. Accordingly, the manufactured glasssubstrate 21 has both high cleanness and superior smoothness.

In the grinding process S15, the surface of the glass plate 21 a isground to a depth of 0.5 nm or greater from the surface of the glassplate 21 a so that the remaining surface layer 27 has a thickness of 3nm or less subsequent to grinding. In this manner, the grinding amountis adjusted to prevent the surface layer 27 from excessively remainingon the surface of the glass plate 21 a subsequent to grinding. Thewashing subsequent to grinding prevents uneven etching of the surfacelayer 27. Thus, a smooth glass substrate 21 is manufactured.

The ingredient ratio of silicon oxide in the surface layer 27 relativeto the ingredient ratio of silicon oxide in the glass composition of theinner portion 26 is greater by more than 1.0 times but less than orequal to 1.2 times. This prevents the chemical resistance of the surfacelayer 27 from being excessively decreased. Thus, the surface of theglass plate 21 is prevented from being rough due to washing.

In the pre-grinding washing process S14, a strong acid solution washingliquid is used. Thus, alkaline earth metal ions or aluminum ions areselectively dissolved from the surface of the glass plate 21 a. Thisobtains the surface layer 27 with a content amount of silicon oxide thatis relatively greater than that of the inner portion 26. Change in thecontact time of the glass plate 21 a with the washing liquid facilitatesadjustment of the chemical resistance of the surface layer 27.

The grinding process S15, which uses a texture machine, is performed bysliding the tape 32 in the circumferential direction of the glass plate21 a. This ensures the formation of the projections 24, which extend inthe circumferential direction. Thus, the glass substrate 21 ismanufactured with a high yield and the occurrence of glide errors isprevented even when the head moves near by.

Examples of the present invention and comparative examples will now bedescribed.

Example 1

A glass plate having a size with a thickness of 0.6 mm, an outerdiameter of 65 mm, and an inner diameter of 20 mm was prepared from analuminosilicate glass sheet. The composition of the aluminosilicateglass sheet was SiO₂ 63 mol %, Al₂O₃ 16 mol %, Na₂O 11 mol %, Li₂O 4 mol%, MgO 2 mol %, and CaO 4 mol %. Then, the pre-grinding washing processS14 was performed. The glass plate was immersed in hydrofluoric acidhaving a concentration of 0.01% for three minutes under a temperature of35° C. and then immersed in a potassium hydroxide solution (KOH) havinga concentration of 0.01% for three minutes under a temperature of 35° C.

The two surfaces of the glass plate were then ground. In the grinding,an abrasive containing diamond abrasion grains was used, and the glassplate was ground for a grinding amount of 2 nm without forming thetexture. After the grinding, a surface layer having a thickness of 2 nmremained in the surface of the glass plate. The glass plate was immersedin a potassium hydroxide solution having a concentration of 1% for threeminutes under a temperature of 35° C. to perform the post-grindingwashing. The glass substrate of example 1 was obtained in this manner.

An AFM was used to measure the Ra on the surface of the glass substrateat ten or more locations. The field of vision was 10 μm×10 μm. Thedeviation rate of Ra was calculated from a totality mean value of RA forall of the measurement locations and an individual mean value of Ra foreach field of vision. The deviation rate is the ratio of the number ofmeasurement locations in which the individual mean value differs by 0.1nm or greater relative to the totality mean value with respect to thenumber of all of the measurement locations. For example, when 10locations on the surface of the glass substrate are measured and theindividual mean value differs from the totality mean value by 0.1 nm orgreater at three locations, the deviation rate is 30%. In other words, ahigher deviation rate of Ra indicates that the surface of the glasssubstrate is rough, and a low deviation rate indicates that the glasssubstrate is smooth. In example 1, the deviation rate of Ra was lessthan or equal to 3%. Further, TDH was 4 nm. Accordingly, the glasssubstrate of example 1 was a glass substrate having satisfactorysmoothness and a low levitation height.

Example 2

A glass substrate was obtained in example 2 through the same method asexample 1 except in that the grinding amount was 4 nm and the thicknessof the surface layer was 1 nm. In example 2, the deviation rate of Rawas less than or equal to 2%, and TDH was 3.5 nm. Accordingly, the glasssubstrate of example 2 was a glass substrate having satisfactorysmoothness and a low levitation height.

Example 3

A potassium hydroxide solution having a concentration of 0.02% was usedwhen performing pre-grinding washing, and a potassium hydroxide solutionhaving a concentration of 2% was used in the post-grinding washing.Otherwise, a glass plate was processed in the same manner as in example1 to obtain a glass substrate in example 3.

Texture Formation Conditions

Material of tape: polyester

Tension of tape: 22.1 N

Velocity of tape: 7.6 cm/min

Pressing force of roller: 30.9 N

Rotation speed of glass plate: 300 rpm

Supply amount of diamond slurry: 20 ml/min

Grain diameter of diamond abrasion grains: 0.2 μm

The Ra deviation rate of the glass substrate was less than or equal to3% and TDH was 2.5 nm. Comparing this with the result of example 1, thedeviation rate of Ra was the same but TDH was smaller. This shows thatthe formation of the texture enables stable manufacturing of asatisfactory glass plate having a low levitation height.

Comparison Example 1

A glass plate was processed in the same manner as in example 1, exceptin that grinding was not performed, to obtain the glass substrate ofcomparative example 1. The thickness of the surface layer was 5 nm. Thedeviation rate of Ra for the glass substrate of comparative example 1was greater than or equal to 15% and TDH was 5 nm. Comparing this to theglass plate of example 1, the deviation rate of Ra increased and the TDHwas greater. Thus, when grinding is not performed, the surface layer isunevenly etched. This decreases smoothness and hinders the decreasing ofTDH and thus is not preferable.

The embodiment and examples may be modified as described below.

As described in the examples, as long as part of the surface layer 27 isremoved, the texture 23 does not have to be formed during grinding. Whenthe texture is not formed during grinding, the grain diameter and shapeof the diamond abrasion grains may appropriately be selected whenforming the texture during grinding to grind the projections 24 having aheight that is less than the thickness of the surface layer 27. Thisfacilitates manufacturing and improves the yield of the glass substrate.

If necessary, the glass plate 21 a may be washed after at least any oneof the disk machining process S11, the edge chamfering process S12, thepolishing process S13, the pre-grinding washing process S14, thegrinding process S15, and the post-grinding washing process S16. Thiswashing includes washing for removing adhered substances, such asabrasion grains, iron particles, and dust, from the surface of the glassplate 21 a, and washing for removing the washing liquid remaining on thesurface of the glass plate 21 a. The acid solution, alkaline solution,and neutral solution described above may be used for the washing.

A chemical strengthening process may be performed between any one of thedisk machining process S11, the edge chamfering process S12, thepolishing process S13, the pre-grinding washing process S14, thegrinding process S15, and the post-grinding washing process S16. In thechemical strengthening process, the surface of the glass plate 21 aundergoes a chemical strengthening treatment to improve the impactresistance characteristic, the vibration resistance characteristic,thermal resistance characteristic, and etc. that are required for aninformation recording medium. In the chemical strengthening treatment,monovalent metal ions contained in the glass composition, such aslithium ions or sodium ions, are ion converted to monovalent metal ionshaving a greater ion radius, such as sodium ions or potassium ions. Thechemical strengthening treatment forms a compression stress layer on thesurface of the glass plate 21 a and chemically strengthens the surface.The chemical strengthening treatment is performed by immersing the glassplate 21 a in a chemical strengthening liquid in which potassium nitrate(KNO₃), sodium nitrate (NaNO₃), silver nitrate (AgNO₃), and etc. areheated and melted. The chemical strengthening treatment is preferablyperformed under a temperature of about 50 to 150° C. lower than thestrain point of the glass material that is used, and more preferably,the temperature of the chemical strengthening liquid is about 350 to400° C.

The glass material of the glass substrates in examples 1 to 3 is analuminosilicate glass of which glass composition contains alkaline earthmetal oxide and aluminum oxides. However, the glass material is notlimited in such manner and soda lime glass, borosilicate glass, orcrystallization glass may be used. The soda lime glass, borosilicateglass, and crystallization glass do not have to include any aluminumoxides or may include just a slight amount of oxides. When using theseglass materials, the surface layer is formed by the alkaline earth metalions of the alkaline earth metal oxides dissolved out of the glasscomposition.

The surface layer 27 does not have to be formed by removing alkalineearth metal ions or aluminum ions and may be formed by removing alkalinemetal ions such as potassium ions, sodium ions, and lithium ions.

In the grinding process S15, any device may be used as long as it rubsthe surface of the glass plate 21 a and grinds off the surface layer 27.When removing the surface layer through grinding, it is preferred that adevice that rubs the surface of the glass plate 21 a in thecircumferential direction, which is the movement direction of the head,be used. This is because TDH may be decreased by grinding the surface ofthe glass plate 21 a in the circumferential direction. The preferreddevice is a scrub machine. A scrub machine is a device that rubs thesurface of the glass plate with a rotatably supported synthetic resinscrub member or foam scrub member (scrub material).

1. A method for manufacturing a glass substrate for a magnetic disk, themanufacturing method comprising: a polishing step for polishing adisk-shaped glass plate made of a multi-component glass materialcontaining silicon oxide to form a polished smooth surface; apre-grinding washing step for washing the polished smooth surface of theglass plate with an acid solution followed by an alkaline solution toform an altered surface layer having an controlled thickness on an innerportion of the glass plate, an ingredient ratio of silicon oxide in thealtered surface layer being different from that in the inner portion; agrinding step for using abrasion grains to grind the altered surfacelayer formed in the pre-grinding washing step such that the alteredlayer having a thickness less than the controlled thickness remains onthe inner portion; subsequent to the grinding step, a post-grindingwashing step for washing the glass plate with a neutral or alkalinewashing liquid to remove a remaining portion of the altered surfacelayer.
 2. The manufacturing method according to claim 1, wherein thepolishing step, the pre-grinding washing step, the grinding step and thepost-grinding washing step are consecutively performed in this order. 3.The manufacturing method according to claim 1, wherein: the grindingstep forms a texture including a projection in the altered layer and theinner portion; and the post-grinding washing step removes a part of theprojection in the altered layer without affecting a remaining part ofthe projection in the inner portion.
 4. The manufacturing methodaccording to claim 1, wherein the pre-grinding washing step includes apre-grinding first washing step using the acid solution to form thealtered surface layer and a pre-grinding second washing step using thealkaline solution to control the thickness of the altered surface layeron the inner portion.
 5. The manufacturing method according to claim 4,wherein the altered surface layer formed in the pre-grinding firstwashing step is removed in a step-by-step manner by consecutive steps ofthe pre-grinding second washing step, the grinding step and thepost-grinding washing step.